Method for reducing noise of a cooking hood and a cooking hood based on such method

ABSTRACT

A method and system for reducing noise of a cooking hood having a body containing a motor and a muffler mounted on an upper section of the body so as to be in fluid connection therewith. A motor active noise reduction unit produces motor reduction noise for reducing motor noise. Likewise, a muffler active noise reduction unit produces a muffler noise reduction signal for reducing muffler noise.

FIELD OF THE INVENTION

[0001] This invention relates to reducing noise of cooking hoods.

BACKGROUND OF THE INVENTION

[0002] Cooking hoods are usually noisy appliances in the kitchen. Theprior art describes efforts to reduce the noise.

[0003] U.S. Pat. No. 5,720,274 entitled “Low-noise vapor exhaust hood”(1998, Brunner et al., in the name of Gaggenau-Werke Haus-undLufttechnik GmbH) discloses a vapor exhaust hood with a housing whichfeatures at least one intake opening equipped at a minimum with onefilter, and features at least one outlet opening, and the insidesurfaces of which are in part lined with a sound-deadening material,with a fan arranged in the housing. Arranged in the housing, at leastopposite each inlet opening of the fan, is sound-absorbing materialwhich from the opposite inlet opening is spaced at least the radius ofthe fan wheel. The sound-absorbing material has a surface which in sizecorresponds at least to a circular surface whose radius matches that ofthe fan wheel. Further measures for noise reduction consist in thearrangement of a vane in the area of the outlet opening, in theapplication of sound-deadening and/or sound-damping material as well asin the elastic mounting of fan and/or fan motor. The invention disclosedin U.S. Pat. No. 5,720,274 thus creates a vapor exhaust hood in which,for one, the number of noise generators is reduced and, for another,inevitable noise is damped and deadened.

[0004] U.S. Pat. No. 5,890,484 entitled “Exhaust system for kitchens”(1999, Yamada Yoshihiro) discloses an exhaust device for a kitchenexhaust system incorporating a range hood, which is capable ofsubstantially reducing generation of noise while efficiently evacuatingfumes from the kitchen without a suction loss. The exhaust devicecomprises a vent box communicating with an exhaust duct, which may behoused in the hood or in a housing box provided above the hood. The ventbox includes a connection enclosure in its upper portion, whichconverges toward the duct. The vent box contains a pair of partitionseach having a tilted plate, which provide a pair of vent routes. Betweenthe partitions is provided a drive motor and in each vent route isprovided a sirocco fan driven by the motor whose blades are convergedtoward the motor.

[0005] U.S. Pat. No. 6,439,839 entitled “Blower” (2002, Song Sung Bae etal., in the name of LG Electronics Inc.) discloses a blower. The blowercomprises an impeller and a scroll housing. The impeller is providedwith a plurality of blades and rotated. The scroll housing guides anddischarges air sucked by the impeller to the outside, and surrounds theimpeller. The expansion angle of the curvature radius of the contour ofthe scroll housing is designed to be less than an expansion angle inconformity with an Archimedic curve in a suction region ranging from acutoff start angle to 160-200° from a reference angle. Additionally, theexpansion angle of the curvature radius of the contour of the scrollhousing is designed to be greater than the expansion angle in conformitywith the Archimedean curve in a discharge region ranging exceeding160-200°.

[0006] U.S. Pat. No. 6,368,062 entitled “Turbo fan for range hood andrange hood storing turbo fan” (2002, Yagami Mototake and Sato Kiyohiko,in the name of Fuji Industrial Co., Ltd) describes that the spring-backstate of the blade press formed from the metallic thin plate isrestricted to form the blade of wing sectional shape strictly inaccordance with the design disclosed in U.S. Pat. No. 6,368,062. Theblade is formed into the wing sectional shape having a hollow inner partwith both sides fixed to the upper plate and the lower plate beingreleased under application of the press forming of the metallic thinplate. The blade is made such that the metallic thin plate having arectangular shape as seen from its top plan view with one side being awing width size is applied with a coining work, a number of lineardeformation segments in parallel with the side of the wing width sizeare properly spaced apart along a side crossing at right angle with theside of the wing width size in side-by-side relation, the directioncrossing at a right angle with the side of the wing width size of themetallic thin plate is formed into the curved surface of predeterminedcurvature and then a transfer of the recovering force generated at eachof the belt-like plates between the linear deformation segments is shutoff at the linear deformation segments so as to restrict influenceagainst the entire metallic thin plate.

[0007] U.S. Pat. No. 5,983,888 entitled “Low noise cooker hood” (1999,Anselmino Jeffery John and Wu Guolian, in the name of WhirlpoolCorporation) discloses a low noise hood having a housing with an airinlet and an air outlet. A first modular device in the form of an intakemuffler with a first air duct passage extending therethrough is mountedwithin the housing near the air inlet. A second modular device in theform of a discharge muffler with a second air duct passage extendingtherethrough is mounted within the housing near the air outlet. Thesecond air duct passage is shaped to prevent a straight unobstructedpassage for airflow through the second passage. A third devicecomprising an air moving device is secured by a mounting arrangementwithin the housing. A first vibration isolator such as a plastic saddleis provided in the mounting arrangement for the air moving device forabsorbing vibrations and a second vibration isolator in the form of aflexible connector is positioned between the air moving device and thesecond air duct passage. A third vibration isolator in a mountingarrangement for the discharge muffler may also be used.

[0008] The above-referenced disclosures employ passive methods for noisereduction. Such passive noise reduction is mainly effective forfrequencies above about 1000 Hz, and may affect airflow in the cookinghood. Also, the passive noise reduction methods and systems disclosedtherein do not reduce noise coming out from the top of the hood.

[0009] JP 6,185,778 entitled “Range Hood” (1994, Takeyama Hiroaki etal., in the name of Matsushita Electric Works Ltd.) describes that thenoise of a sirrocco fan or a discharge duct mounted at a rear of the fanis propagated toward the vicinity of a cooking range. The propagatednoise is sensed by a sensor microphone, subjected to inversionprocessing or adaptive signal processing by a signal processor amplifierto direct an added sound, the added sound is radiated by a loudspeaker,and waves of the sound are interfered with those of the noise to besilenced. A voice signal of a sound source input via an audio jack issent by the loudspeaker via the amplifier to allow a person near a hoodbody to listen to music or broadcasting.

[0010] There is a need in the art to provide for an improved method forreducing noise of a cooking hood and a cooking hood based on suchmethod.

SUMMARY OF THE INVENTION

[0011] It is the object of the invention to provide a method forreducing noise of a cooking hood having a body containing a motor and amuffler mounted on an upper section of the body so as to be in fluidconnection therewith, the method comprising:

[0012] providing or using in association with the motor a motor activenoise reduction unit capable of producing a motor reduction noise forreducing motor noise; and

[0013] providing or using in association with the muffler a muffleractive noise reduction unit capable of producing a muffler reductionnoise for reducing muffler noise.

[0014] It is another object of the invention to provide for a cookinghood having a body containing a motor and further containing a mufflermounted on an upper section of the body portion so as to be in fluidconnection therewith, the cooking hood comprising:

[0015] a motor active noise reduction unit mounted in association withthe motor; and

[0016] a muffler active noise reduction unit mounted in association withthe muffler.

[0017] Still further, it is the object of the invention to provide for amotor unit for a cooking hood, said motor unit containing a motor and amotor active noise reduction unit integral therewith.

[0018] Yet further, it is the object of the invention to provide for amuffler unit for a cooking hood, said muffler unit containing a mufflerand a muffler active noise reduction unit integral therewith.

[0019] Still further, it is the object of the invention to provide for amotor active noise reduction unit for reducing motor noise for use in acooking hood, said motor active noise reduction unit comprising:

[0020] at least one microphone for receiving the motor noise;

[0021] a speaker capable of producing motor reduction noise for reducingthe motor noise; and

[0022] an electronic circuit connecting the speaker and the at least onemicrophone, for adjusting the motor reduction noise in accordance withthe motor noise.

[0023] It is also the object of the invention to provide for a muffleractive noise reduction unit for reducing muffler noise for use in acooking hood, said active noise reduction unit comprising:

[0024] at least one microphone for receiving the muffler noise;

[0025] at least one speaker capable of producing muffler reduction noisefor reducing the muffler noise; and

[0026] an electronic circuit connecting the at least one speaker and theat least one microphone, for adjusting the muffler reduction noise inaccordance with the muffler noise.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to understand the invention and to see how it may becarried out in practice, a preferred embodiment will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

[0028]FIG. 1 illustrates a cooking hood according to one embodiment ofthe invention.

[0029]FIG. 2 illustrates the motor active noise reduction unit accordingto one embodiment of the invention.

[0030]FIG. 3 illustrates the motor active noise reduction unit accordingto another embodiment of the invention, employing at least twomicrophones.

[0031]FIG. 4 illustrates the muffler according to one embodiment of theinvention.

[0032]FIG. 5 illustrates an interference-preventing electronic circuitaccording to one embodiment of the invention.

[0033]FIG. 6 illustrates an emulation circuit emulating the noise at thefar field, according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0034] In the following description, components that are common todifferent embodiments are referenced by identical reference numerals.

[0035]FIG. 1 illustrates a cooking hood 101 according to one embodimentof the invention. The cooking hood is composed of a body 102 and amuffler 103 mounted on an upper section of the body 102 and in fluidconnection therewith. The body 102 is surrounded by body walls. Themuffler 103 has a wall surrounding a hollow space of the muffler, fromwhich the air blows out of the cooking hood. A motor 104 within the body102 moves air from an air inlet 105 at the bottom of the body 102, to anair outlet 106 at the top of the body. At least one air opening 107formed in a casing of the motor 104 allows air to enter into the motor104. The motor 104 circulates air entering through the openings 107 tothe air outlet 106, creating air turbulence constituting a dominantnoise source within the cooking hood 101. The air is then circulated outof the cooking hood through the muffler 103. The noise propagates out ofthe cooking hood, for example through the air inlet 105, through thecooking hood's body 102 or through the air outlet 106 and muffler 103.The noise carried out through the air inlet 105 is referred to herein asmotor noise, and it has a typical dominant spectrum of up to about 5000Hz. The noise carried out through the air outlet 106 and muffler 103 isreferred to herein as muffler noise.

[0036] In the cooking hood 101 there is a combination of passive noisereduction mechanisms, which taken in isolation are familiar to thoseversed in the art. For example, the air inlet 105 is padded with layersof absorbing material 108 and 109, which together constitute air inletpadding for reducing noise by passively absorbing high frequencies, e.g.frequencies higher than about 1000 Hz. Passive noise absorbing materialsare well known in the art, suitable examples being polyester andmelamine. However, known passive noise reduction methods show poorperformance when reducing low frequency noise, and the inventionproposes a combination of active and passive noise reduction techniques,in order to improve the performance at both low and high frequencies.Active noise reduction, according to different embodiments of theinvention, is effected by generating a reduction noise, which interactswith the noise and reduces its intensity. The reduction noise is mostpreferably of opposite phase and equal amplitude to the noise created bythe cooking hood, thereby reducing the noise signal.

[0037] As seen in FIG. 1, a motor active noise reduction unit 110 isplaced within the cooking hood, in association with the motor 104. Themotor active noise reduction unit 110 is positioned close to the motorin order to detect and reduce the motor noise generated therein. Thus,in the figure the motor active noise reduction unit 110 is locatedbeneath the motor 104 and it is padded with the noise absorbing material108. The motor active noise reduction unit 110 is placed so as not tointerfere with airflow through the air inlet 105, and is used to reducelow frequency noise created by the air turbulence at the motor andcarried out of the cooking hood through the air-inlet 105.

[0038] As further seen in FIG. 1, a muffler active noise reduction unit111 is placed in association with the muffler 103, and is used to reducelow frequency muffler noise carried through the air outlet 106 and themuffler 103. However, the muffler active noise reduction unit 111 neednot necessarily be confined inside a housing, and can be distributed inthe muffler's hollow space, as will be described later with reference toFIG. 4.

[0039] There can exist a physical connection between the cooking hood'sbody 102 and the motor 104. It is at least partly because of thisphysical connection that motor vibrations create vibrations of the body102. The body vibrations are converted to acoustical noise. This noisemay become dominant when all other noise sources in the cooking hood arereduced. In order to reduce the noise created by the motor vibrations,dampers 112 that are suitable for the motor speed and weight can beused.

[0040] Besides the air inlet 105, the air outlet 106 and the muffler103, noise created by air turbulence in the motor 104 can emanatethrough the cooking hood's body 102. This noise can lower the totaleffect of noise reduction performed passively and/or actively by themotor active noise reduction unit 110, by the muffler active noisereduction unit 111 and by the passive mechanisms described above. Inorder to reduce the noise emanating from the cooking hood's body 102,the body 102 can be built of acoustically insulating material. However,normally the weight and the size of the passive materials needed foreffective insulation increase as the insulation requirement increases,and optimization is required in order to keep the cooking hood's body aslight-weight and small as possible while reducing the noise emanatingtherefrom to negligible intensities.

[0041] The optimally reduced noise emanating from the walls of the body102 is expected to be lower by about 10 dB or more than the reducednoise that emanates from the air-inlet 105. It is also expected to belower by about 10 dB or more than the reduced noise that emanates fromthe air-outlet 106 and the muffler 103. By “reduced” noise is meant thatnoise that emanates from the body 102, the air-inlet 105 and theair-outlet 106 when passive and active noise reduction mechanisms areactivated. It is possible to measure the reduced intensity of the noiseemanating from different locations of the cooking hood 101 using knownmethods for acoustic power measurement. Several of such methods aredescribed by acoustic standards such as ECMA-160 (second editionDecember 1992).

[0042]FIG. 2 illustrates the motor active noise reduction unit 110according to one embodiment of the invention. The outer walls of themotor active noise reduction unit 110 are made of material transparentto sound, such as a dense grid of metal, enabling sound waves to passthrough. According to the described embodiment, the motor active noisereduction unit 110 includes a hollow space, enclosed within inner walls,forming an enclosure 201. A speaker 202 is located inside the enclosure201 and produces a motor reduction noise for reducing noise generated bythe motor. According to the described embodiment, a microphone 203 islocated at the center top, close to the motor 104 (see also FIG. 1), inorder to receive the motor noise. The microphone 203 produces then anelectric signal referred to as a “motor noise signal” that correspondsto the motor noise. An electronic circuit 204 connects the microphone203 to the speaker 202 in order to calculate a motor noise reductionsignal to be fed to the speaker 202, based on the motor noise signalpicked up by the microphone 203. The speaker 202 generates the motorreduction noise in accordance to the motor noise reduction signal itreceives.

[0043] The enclosure 201 has at least one noise-transparent inner-wall,such as the bottom wall. The other inner-walls are noise opaque.Speakers manufactured by currently available technology typicallygenerate acoustical waves having opposite phases from opposite sides ofthe speaker's membrane. According to the described embodiment, the noiseopaque inner-walls acoustically insulate the speaker, preventing thesignals with the opposite phases from propagating out of the speakerenclosure.

[0044] As already noted with regard to FIG. 1, the walls of the motoractive noise reduction unit (i.e. the outer walls) are padded with noiseabsorbing material 108. The noise absorbing material padding 108,together with the noise absorbing material padding 109 passively absorbhigh frequency noise created by the motor and leaving the cooking hoodthrough the air inlet.

[0045] As known in the art, air turbulence may develop around objectspositioned in the airflow route. This air turbulence is referred to aslocal turbulence. Local turbulence evolving around the motor activenoise reduction unit 110 may affect the acoustic noise received by themicrophone 203.

[0046]FIG. 3 illustrates an alternative embodiment that copes with localturbulence, reducing its effect on the acoustic noise received by themotor active noise reduction unit 110. The motor active noise reductionunit 110 employs at least two microphones 301 a, . . . , 301 n. Themicrophones 301 a, . . . , 301 n together form a microphone section 302.The microphones are positioned as pairs disposed symmetrically onopposite sides of the motor active noise reduction unit 110. Forexample, there are shown four microphones designated 301 a, 301 b, 301 cand 301 d, where 301 a and 301 c form a first pair, and are disposedsymmetrically on opposite sides of the motor active noise reduction unit110. The same applies with regard to the microphones 301 b and 301 d,which form a second pair. Electronic circuits 303 a, . . . , 303 nelectronically connect the at least two microphones 301 a, . . . , 301n, whereupon they all summed at a summation point 304. Electroniccircuit 204 exists also between the at least two microphones 301 a, . .. , 301 n and the speaker 202 in order to calculate the motor noisereduction signal and therefore the motor reduction noise generated bythe speaker 202 with the motor noise signal picked up by the microphones301 a, . . . , 301 n.

[0047] The microphones are gain- and phase-calibrated by means of theelectronic circuits 303 a, . . . , 303 n, to have the same gain andphase. According to the described embodiment, the microphones areselected to have matched phase within a frequency range up to about 1000Hz. The electronic circuits 303 a, . . . , 303 n are used to calibratethe sensitivity of all microphones 301 a, . . . , 301 n so as to beidentical, by changing the analog gain of each microphone. Usingmultiple (at least two) microphones increases the signal to noise ratioof microphone section 302 as is known in the art. This way the motoractive noise reduction unit can detect noise even when high-speedairflow exists, reducing the acoustic effect of local turbulence. Gainand phase calibration per se is well known in the art. See for exampleU.S. Pat. No. 4,454,597, U.S. Pat. No. 4,956,867 and U.S. Pat. No.5,574,824, disclosing methods for gain and phase calibration as well asmethods to increase the signal to noise ratio in the presence of anumber of microphones.

[0048]FIG. 4 illustrates the muffler 103 according to a non-limitingembodiment of the invention shown with a round cross-section although itshould be notified that the muffler's cross-section can be of any form.Airflow through the muffler 103 is directed from the muffler's inlet 402to a muffler outlet 403, therefore being asymmetrical. As explained withreference to FIG. 1, the muffler 103 is mounted on an upper section ofthe body 102, and air blows out of the cooking hood blows through themuffler's hollow space. It was also explained that the air blows out ofthe cooking hood's body through the air outlet 106. Therefore, the airoutlet 106 and the muffler's inlet 402 can be considered as equivalents.

[0049] As also explained previously, the muffler noise emanates from thecooking hood's air outlet 106 through the muffler 103. The mufflerperforms both passive and active noise reduction, in order to reduce thenoise emanating therefrom. In order to perform passive noise reduction,the muffler's walls are padded or are built of acoustically absorbingmaterial 401, reducing frequencies above about 1000 Hz. Frequenciesbelow about 1000 Hz are reduced by the muffler active noise reductionunit 111 described below.

[0050] According to the asymmetrical muffler airflow, the muffler activenoise reduction unit 111 is also asymmetrically distributed along themuffler 103. At least two microphones 404 a, . . . , 404 n are disposedaround the muffler's walls internal surface near the muffler inlet 402,forming a muffler microphone section 405. Each of the microphones 404 a,. . . , 404 n is connected by means of respective micro-phone electroniccircuits 406 a, . . . , 406 n to a summation point 407, summing therespective outputs of the microphones 404 a, . . . , 404 n. The muffler103 also includes a speaker 408 mounted on the internal surface of themuffler's wall, and connected by an electronic circuit 409 to themuffler microphone section 405 via the summation point 407. The speaker408 receives a muffler noise reduction signal and produces acorresponding muffler reduction noise, such as noise of opposite phaseand equal amplitude to the noise emanated through the muffler, forreducing noise generated thereby.

[0051] In variations of the above embodiment multiple speakers may belocated around the internal periphery of the muffler and a singlemicrophone may likewise be employed. In the case where a single speakeris used, it may be displaced horizontally from the position shown inFIG. 4 so that it is located at the center of the muffler'scross-section instead of on the internal surface of the muffler's wall.Likewise, when a single microphone is used it, too, may be displacedhorizontally from the position of the muffler microphone section 405shown in FIG. 4 so that it is located at the center of the muffler'scross-section instead of on the internal surface of the muffler's wall.

[0052] As is known in the art, as the number of microphones 404 a, . . ., 404 n increases, the signal to noise ratio of the microphone section405 increases. An exemplary embodiment uses 8-12 microphones, but thisis not limiting and other numbers of microphones may be employed. Thisway the muffler active noise reduction unit 111 can detect noise evenwhen high-speed airflow exists, reducing the acoustical effect of thelocal turbulence at the microphone section 405. The at least twomicrophones 404 a, . . . , 404 n are symmetrically arranged around aninternal surface of the muffler wall. The microphone electronic circuits406 a, . . . , 406 n perform gain- and phase-calibration of themicrophones 404 a, . . . , 404 n, in order for them to have the samegain and phase. According to this embodiment, the microphones areselected to have matched phase within a frequency range up to about 1000Hz. The microphone electronic circuits 406 a, . . . , 406 n are used tocalibrate the microphones 404 a, . . . , 404 n, by changing the analoggain of the microphones so that they are identical.

[0053] The speaker 408 normally generates a muffler reduction noiseoriented towards the interior and exterior of muffler 103. In order toprevent the reduction noise oriented towards the muffler exterior fromemanating outside the muffler's wall, the speaker's posterior side ismade of or is padded with insulating material.

[0054] In any of the embodiments so far described, if the two activenoise reduction units (the motor active noise reduction unit 110 and themuffler active noise reduction unit 111) are located close to eachother, interference may occur. A reduction noise generated by one unitmay affect the other, reducing its performance of active noisereduction. Thus, when the microphone 203 for example measures the motornoise, it measures also a component of the muffler reduction noisereaching the motor active noise reduction unit, i.e. the microphone 203measures a cumulative characteristic of motor noise together with acomponent of the muffler reduction noise. In response to measuring thecumulative characteristic of motor noise together with a component ofthe muffler reduction noise, the microphone 203 produces a signalreferred to as a cumulative motor noise signal. In the same manner,microphones 404 a, . . . , 404 n measure a cumulative characteristic ofmuffler noise together with a component of the motor reduction noise,and produce a signal referred to as a cumulative muffler noise signal.

[0055] Therefore, in the case that interference exists, it is desirableto reduce the component of the muffler reduction noise from thecumulative characteristic of motor noise together with a component ofthe muffler reduction noise measured by the microphone 203 of the motoractive noise reduction unit 110. It is likewise desirable to reduce thecomponent of the motor reduction noise from the cumulativecharacteristic of muffler noise together with a component of the motorreduction noise measured by the microphones 404 a, . . . , 404 n of themuffler active noise reduction unit 111. This correction must be donenotwithstanding that the motor reduction noise and the muffler reductionnoise both change during the operation of the cooking hood.

[0056]FIG. 5 illustrates an interference-preventing electronic circuit501 according to one embodiment of the invention. The microphone 203 ofthe motor active noise reduction unit 110 is positioned a constantdistance from the speaker 408 generating the muffler reduction noise.Thus, for the purpose of interference preventing the medium throughwhich the muffler reduction noise passes on its way to the microphone203 can also be considered constant. Therefore, the way this constantmedium affects the passing muffler reduction noise is also constant.This constant effect can be measured offline (i.e. when the cookinghood's motor is switched off) and represented by a muffler-motortransfer function 502 in ways known to those versed in the art. Themuffler-motor transfer function 502 is coupled between the speaker 408and the microphone 203 via a muffler-motor subtraction point 504.

[0057] The muffler-motor transfer function 502 receives the mufflernoise signal fed into the speaker 408 of the muffler noise reductionunit 111. Representing the constant medium effect on the component ofmuffler reduction noise, the muffler-motor transfer function 502calculates the component of muffler noise reduction signal correspondingto this component of muffler reduction noise.

[0058] The muffler-motor subtraction point 504 subtracts the componentof muffler noise reduction signal from the cumulative motor noise signalproduced by the microphone 203. Thus the subtraction yields an effectivemotor noise signal component that correspond to the effective motornoise. This effective motor noise signal component is used, instead ofthe cumulative motor noise signal, for the generation of the motor noisereduction signal, and its corresponding motor reduction noise, thereforereducing the effective motor noise and not the cumulative motor noise.

[0059] Likewise, the effect on the motor reduction noise generated bythe speaker 202 and reaching microphones 404 a, . . . , 404 n may alsobe considered constant, and can be measured and represented by amotor-muffler transfer function 503. The motor-muffler transfer function503 connects the speaker 202 to the microphones 404 a, . . . , 404 n viaa motor-muffler subtraction point 505. The motor-muffler subtractionpoint 505 subtracts the component of motor noise reduction signal fromthe cumulative muffler noise signal produced by the microphones 404 a, .. . , 404 n. Thus the subtraction yields an effective muffler noisesignal component corresponding to the effective muffler noise. Thiseffective muffler noise signal component is used, instead of thecumulative muffler noise signal, for the generation of the muffler noisereduction signal, and its corresponding muffler reduction noise,therefore reducing the effective muffler noise and not the cumulativemuffler noise.

[0060] If the muffler-motor transfer function 502 is known, and thechanging muffler reduction noise is measured, it is possible tocalculate the component of the muffler noise reaching the microphone203. In the same manner, if the motor-muffler transfer function 503 isknown, and the changing motor reduction noise is measured, it ispossible to calculate the component of the motor reduction noisereaching the microphones 404 a, . . . , 404 n of the muffler activenoise reduction unit 111.

[0061] As explained previously with reference to the muffler-motortransfer function 502 and the motor-muffler transfer function 503, itshould be noted that in a non-limiting manner the components of themuffler and motor reduction noise can be measured offline to constitutea predetermined component of muffler reduction noise and a predeterminedcomponent of motor reduction noise. The predetermined muffler and motorreduction noise can then be subtracted to yield the effective mufflerand motor signal components. This can be done by means of using transferfunctions, as previously described with reference to FIG. 5.

[0062] Further to what has been described so far, also known to thoseversed in the art are active noise reduction units having microphones,speakers and electrical cancellation circuits, designed to reduce noiseat the microphone position. Noise at the microphone location is referredto as the “near field”. However, noise reaches also to locations remotefrom the motor, referred to as the “far field”. Known active noisereduction units, which are designed to reduce noise at the microphonelocation, normally cannot reduce the far field noise, which is locatedfar from the motor, and therefore far from the microphone.

[0063]FIG. 6 illustrates an emulation circuit 600 emulating the noise atthe far field, according to one embodiment of the invention. Theemulation circuit 600 includes a microphone 601 connected via amicrophone electronic connection 602 to a microphone transfer function603. The emulation circuit includes also a speaker 604 connected via aspeaker electronic connection 605 to a speaker transfer function 606.The microphone transfer function 603 and the speaker transfer function606 are connected by a summation point 607 where the emulation circuit'soutput is computed. The emulation circuit's output is directed throughthe output electronic connection 608.

[0064] The emulation circuit is suitable for both the motor noisereduction unit 110 and the muffler active noise reduction unit 111.Therefore, in the motor noise reduction unit 110, where the emulationcircuit is part of the electronic circuit 204, the microphone 601 iseither analogous to the microphone 203 as referenced in FIG. 2, or tomicrophones 301 a, . . . , 301 b as referenced in FIG. 3, and thespeaker 604 is analogous to the speaker 202. In the muffler noisereduction unit 111, where the emulation circuit is part of theelectronic circuit 409, the microphone 601 is analogous to themicrophones 404 a, . . . , 404 n and the speaker 604 is analogous to thespeaker 408.

[0065] It should also be noted that when the emulation circuit isapplied to the motor active noise reduction unit 110 and with referenceto FIG. 6, the term “noise” refers to “motor noise”, the term “noisesignal” to “motor noise signal”, the term “noise reduction signal”refers to “motor noise reduction signal” and the term “reduction noise”refers to “motor reduction noise”. When applying to the muffler noisereduction unit 111, the terms refer to “muffler noise”, “muffler noisesignal”, muffler noise reduction signal” and “muffler reduction noise”respectively.

[0066] The microphone 601 has to process the received acoustic signals(the noise) as if it were positioned in the far field at a far pointwhere the noise is expected to be reduced to its minimal level. This farpoint is known to those versed in the art as an error point. Theprocessing is carried out in order to tune the speaker 604 accordingly,so as to reduce the noise in the far field. The microphone transferfunction 603 represents the environment between the microphone and theerror point. Thus, applying the microphone transfer function 603 to anoise received by the microphone 601 emulates the noise that would havebeen received by a microphone positioned at the error point, if such amicrophone would have been positioned thereat.

[0067] The speaker 604 produces noise (reduction noise according to thedescribed embodiment) which reach the error point reducing noisethereat. A speaker transfer function 606 describes the environmentbetween the speaker and the error point. By applying the speakertransfer function 606 to the noise reduction signal that is fed intospeaker 604, it is possible to estimate the reduction noise that wouldhave been received by a microphone positioned at the error point, ifsuch a microphone would have been positioned thereat.

[0068] It should be noted that the term “emulated noise” (correspondingboth to “emulated motor noise” and to “emulated muffler noise”) is usedwith reference to the noise corresponding to the noise signal receivedby the microphone 601 after being processed by the microphone transferfunction 603. The noise signal, processed by the microphone transferfunction 603 is referred to as an “emulated noise signal”. If the noisereduction signal is fed to the speaker transfer function 606 beforebeing fed to the speaker 604 it is referred to as an “emulated noisereduction signal”. The noise corresponding to the emulated noisereduction signal generated by the speaker is referred to as “emulatedreduction noise” (corresponding to “emulated motor reduction noise” andto “emulated muffler reduction noise”).

[0069] The noise at the error point is reduced to its minimal level whenthe sum (as computed by a summation point 607) of the emulated noise andthe emulated reduction noise (i.e. when the sum of the emulated noisesignal and the emulated noise reduction signal) is minimal. Therefore,in order to receive a minimal noise level at the error point when thetwo transfer functions and the noise are known, the emulation circuitcomputes a required noise reduction signal (i.e. the emulated noisereduction signal), and feeds this emulated noise reduction signal to thespeaker 604, which generates the corresponding emulated reduction noise.Therefore, in the muffler noise reduction unit 111 (see FIG. 4), forexample, the output signal of the emulation circuit is fed by the outputelectronic connection 608 to the electronic circuit 409, effectingadjustments as computed at the summation point 608.

[0070] The microphone transfer function 603 and the speaker transferfunction 606 can be measured by many methods known in the art such as,for example, the Least Mean Square (LMS) method described in U.S. Pat.No. 6,389,440, U.S. Pat. No. 5,909,425 and U.S. Pat. No. 4,977,591 whosecontents are incorporated herein by reference.

[0071] According to one embodiment of the invention, the microphonetransfer function 603 and the speaker transfer function 606 can bemeasured offline (i.e. when switched off) and at any location, e.g. at afactory where the cooking hood is manufactured. As was explained above,the transfer functions 603 and 606 both represent the environment of thecooking hood. The environment is specific to the site and time ofmeasurement. However, when moving the cooking hood to a differentlocation, such as a kitchen, the environment changes. The microphone601, the speaker 604 and the motor 104 are positioned close to eachother. Because of this close position the environment's changing effecton the reduction noise transmitted from speaker 604 to the error pointis approximately equal to its effect on the expected noise at the errorpoint when measured by the microphone 601. Therefore, when the cookinghood is moved to a different position or location, the environmentchanges but as the microphone 601, the speaker 604 and the cooking hood(being a noise source) are positioned close to each other, themicrophone transfer function 603 and the speaker transfer function 606may be expected to change equally, and represent the environment'seffect on the noise. That is, the noise arriving to the error point fromthe cooking hood changes in the same manner as does the reduction noise,thus compensating for the change. Thus, effecting noise reduction at theerror point is expected be effective regardless of the environment wherethe cooking hood is located. This allows measuring the acousticaltransfer functions offline and at any location and even moving thecooking hood to different locations with a minimal effect on the farfield active noise reduction.

1. A method for reducing noise of a cooking hood having a bodycontaining a motor and a muffler mounted on an upper section of the bodyso as to be in fluid connection therewith, the method comprising:providing or using in association with the motor a motor active noisereduction unit capable of producing a motor reduction noise for reducingmotor noise; and providing or using in association with the muffler amuffler active noise reduction unit capable of producing a mufflerreduction noise for reducing muffler noise.
 2. The method according toclaim 1, wherein the motor reduction noise is in the same amplitude andopposite phase to the motor noise.
 3. The method according to claim 1,wherein the muffler reduction noise is in the same amplitude andopposite phase to the muffler noise.
 4. The method according to claim 1,wherein the muffler active noise reduction unit is adapted to producethe muffler reduction noise by: determining a component of the motorreduction noise reaching the muffler active noise reduction unit; andadjusting the muffler active noise reduction unit so as to reduce aneffective muffler noise corresponding to noise sensed by the muffleractive noise reduction unit less said component of the motor reductionnoise.
 5. The method according to claim 4, wherein the component of themotor reduction noise is predetermined.
 6. The method according to claim4, wherein the component of the motor reduction noise is determined by amotor-muffler transfer function.
 7. The method according to claim 5,wherein the predetermined component of the motor reduction noise ismeasured offline.
 8. The method according to claim 1, wherein the motoractive noise reduction unit and the muffler active noise reduction unitare mounted with sufficient mutual separation that interference betweenthe motor active noise reduction unit and the muffler active noisereduction unit is negligible.
 9. The method according to claim 1,wherein the motor active noise reduction unit is adapted to produce themotor reduction noise by: determining a component of the mufflerreduction noise reaching the motor active noise reduction unit; andadjusting the motor active noise reduction unit so as to reduce aneffective motor noise corresponding to noise sensed by the motor activenoise reduction unit less said component of the muffler reduction noise.10. The method according to claim 9, wherein the component of themuffler reduction noise is predetermined.
 11. The method according toclaim 9, wherein the component of the muffler reduction noise isdetermined by a muffler-motor transfer function.
 12. The methodaccording to claim 10, wherein the predetermined component of themuffler reduction noise is measured offline.
 13. The method according toclaim 1, wherein the motor active noise reduction unit is adapted toproduce the motor reduction noise by: determining an emulated motornoise at an error point; and adjusting the motor reduction noise toyield emulated motor reduction noise so as to reduce the emulated motornoise.
 14. The method according to claim 13, wherein the emulated motornoise is determined by a microphone transfer function.
 15. The methodaccording to claim 13, wherein the emulated motor reduction noise isadjusted by a speaker transfer function.
 16. The method according toclaim 1, wherein the muffler active noise reduction unit is adapted toproduce the muffler reduction noise by: determining an emulated mufflernoise at an error point; and adjusting the muffler reduction noise toyield emulated muffler reduction noise so as to reduce the emulatedmuffler noise.
 17. The method according to claim 16, wherein theemulated muffler noise is determined by a microphone transfer function.18. The method according to claim 16, wherein the emulated mufflerreduction noise is adjusted by a speaker transfer function.
 19. Acooking hood having a body containing a motor and further containing amuffler mounted on an upper section of the body so as to be in fluidconnection therewith, the cooking hood comprising: a motor active noisereduction unit mounted in association with the motor; and a muffleractive noise reduction unit mounted in association with the muffler. 20.The cooking hood according to claim 19, wherein the motor is mounted ona substantially hollow base and the motor active noise reduction unit ismounted in the base.
 21. The cooking hood according to claim 19, whereinthe muffler includes a hollow space and the muffler active noisereduction unit is distributed in the hollow space.
 22. A motor unit fora cooking hood, said motor unit containing a motor and a motor activenoise reduction unit integral therewith.
 23. The motor unit according toclaim 22, wherein the motor active noise reduction unit comprises: atleast one microphone for receiving motor noise; a speaker capable ofproducing a motor reduction noise for reducing the motor noise; and anelectronic circuit connecting the speaker and the at least onemicrophone, for adjusting the motor reduction noise in accordance withthe motor noise.
 24. The motor unit according to claim 23, wherein themotor active noise reduction unit further comprises: an emulationcircuit having respective inputs connected to the speaker and to eachmicrophone and having an output connected to the electronic circuit, forreducing noise at a predetermined error point.
 25. A muffler unit for acooking hood, said muffler unit containing a muffler and a muffleractive noise reduction unit integral therewith.
 26. The muffler unitaccording to claim 25, wherein the muffler active noise reduction unitcomprises: at least one microphone for receiving muffler noise; at leastone speaker capable of producing a muffler reduction noise for reducingthe muffler noise; and an electronic circuit connecting the at least onespeaker and the at least one microphone, for adjusting the mufflerreduction noise in accordance with the muffler noise.
 27. The mufflerunit according to claim 26, wherein the muffler active noise reductionunit further comprises: an emulation circuit having respective inputsconnected to the at least one speaker and to the at least one microphoneand having an output connected to the electronic circuit, for reducingnoise at a predetermined error point.
 28. The muffler unit according toclaim 26, wherein the at least one speaker is mounted on an internalsurface of a muffler's wall.
 29. The muffler unit according to claim 26,wherein one of the at least one speaker is centrally disposed inside themuffler.
 30. The muffler unit according to claim 27, wherein the atleast one speaker is mounted on an internal surface of a wall of themuffler.
 31. The muffler unit according to claim 27, having a singlespeaker disposed centrally inside the muffler.
 32. A motor active noisereduction unit for reducing motor noise for use in a cooking hood, saidmotor active noise reduction unit comprising: at least one microphonefor receiving the motor noise; a speaker capable of producing motorreduction noise for reducing the motor noise; and an electronic circuitconnecting the speaker and the at least one microphone, for adjustingthe motor reduction noise in accordance with the motor noise.
 33. Themotor active noise reduction unit according to claim 32, wherein themotor active noise reduction unit further comprises: an emulationcircuit having respective inputs connected to the speaker and to the atleast one microphone and having an output connected to the electroniccircuit, for reducing noise at a predetermined error point.
 34. Amuffler active noise reduction unit for reducing muffler noise for usein a cooking hood, said muffler active noise reduction unit comprising:at least one microphone for receiving the muffler noise; at least onespeaker capable of producing muffler reduction noise for reducing themuffler noise; and an electronic circuit connecting the at least onespeaker and the at least one microphone, for adjusting the mufflerreduction noise in accordance with the muffler noise.
 35. The muffleractive noise reduction unit according to claim 34, wherein the muffleractive noise reduction unit further comprises: an emulation circuithaving respective inputs connected to the at least one speaker and tothe at least one microphone and having an output connected to theelectronic circuit, for reducing noise at a predetermined error point.